On 3/3/14 2:18 AM, Hal Murray wrote:
Junk crystals are good thermometers. Ballpark is 1 ppm/degree-C
[email protected] said:
So does this mean I can epoxy a sandstone power resister to a junk crystal
and keep the frequency exactly perfect by varying the power in the resister?
Sure, for some values of "perfect" and such.
I've occasionally thought about building something like this, just for the
hell of it to see what happens and/or what I learn, and or how good I/we can
get on a low budget.
I think there are two problem areas. One is sensors and control algorithms.
The other is board layout.
Where is the sweet spot on complexity vs accuracy? I'm looking for
science-fair level of goodness rather than super-expensive to get another 0
or two.
What's the best low-cost way to measure temperature? Many of the obvious
choices are only good to 0.1 C. That's great if you are trying to measure
room temperature or or want to keep your CPU from melting, but it's probably
leaving a lot on the table if you are interested in the frequency from a
crystal.
My straw man would be a thermistor and OP-Amp feeding into the ADC on your
favorite uProc. Maybe the other side of a bridge would be adjustable.
A number of microcontrollers have onchip temperature sensors (Freescale
Kinetis, for instance). If the controller were bonded to the crystal
housing, that might be enough coupling.
Could you hold 0.1 or 0.001 degree? the chip has a 16 bid ADC, although
I wouldn't trust the bottom bit or two because of noise. But in any case
1 LSB is 3.3/64k or about 50 microvolts. The temperature sensor slope
is 1.715 mV/C, so that's in the 0.03 C/LSB range.. On a good day, you
*might* be able to hold 0.1 degree, assuming there's no systematic errors.
How much power do you need to keep things warm? I'm assuming something like
a watt or 2 with something like a PWM from the uProc.
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